Information Storage Method and Information Storage Medium with Increased Storage Density by Multi-Bit Coding

ABSTRACT

The invention relates to a method for storage of information and to an information storage medium with increased storage density by multi-bit coding.

INTRODUCTION

The invention relates to a method for storage of information and to aninformation storage medium with increased storage capacity.

With increasing amounts of data being generated day after day improvedstorage techniques are of utmost importance. It is, in particular,important to increase the storage capacity for data per area or perstorage device in order to offer more efficient ways of data storage.Over decades, CDs, DVDs and BluRay Discs were important digitalinformation storage media with the increased storage capacity developedover time being caused by the size of the structures on the one hand andthe wavelength being used for decoding on the other hand.

There is, however, a need to provide an improved method for storage ofinformation as well as an improved information storage medium in termsof storage capacity. The present invention offers different approacheswhich, either alone or in combination, may achieve this object.

SUMMARY

According to a first aspect, the present invention relates to a methodfor storage of information comprising the steps of providing asubstrate; and creating a plurality of recesses in a surface of thesubstrate by using a laser and/or a focused particle beam in order toencode information on the substrate; wherein the plurality of recesseshave different shapes and/or sizes and wherein each shape and/or sizecorresponds to a predefined value of information. In other words, thepresent invention uses a third dimension by not only using a specificspot (e.g., x- and y-dimension) in order to encode information (due to arecess being present or not being present), but in addition uses a shapeand/or size of each recess to encode additional information. Forexample, various circular recesses of different diameters may be used toencode different values of information. Alternatively, ellipsoidalrecesses having different orientations (corresponding to recesses ofdifferent shapes) may be used to encode values of information. Theseconcepts may even be combined by using, e.g., ellipsoids having bothdifferent orientations and different sizes in order to further increasestorage capacity.

Optionally, the plurality of recesses may also have different depthswhich each depth also corresponding to a predefined value ofinformation. The latter concept has been described in great detail inPCT (Int'l) Application Publication No. WO 2022/002418 for ceramicsubstrates. This document is fully incorporated herein by reference, inparticular with regard to any disclosure therein explaining anddetailing how to encode information by means of recess depth. As will beappreciated by the skilled person, this concept may also be utilized forsubstrates other than ceramic substrates.

The concept of this first aspect of the present invention may also beemployed for a stack of material layers. Accordingly, the presentinvention further relates to a method for storage of informationcomprising the steps of providing a substrate; coating the substratewith a layer of a second material different from the material of thesubstrate; and creating a plurality of recesses in a surface of thelayer of the second material by using a laser and/or a focused particlebeam in order to encode information in the layer of the second material;wherein the plurality of recesses have different shapes and/or sizes andwherein each shape and/or size corresponds to a predefined value ofinformation. Again, the plurality of recesses may also have differentdepths with each depth corresponding to a predefined value ofinformation. The coated substrate may optionally be tempered beforeand/or after information encoding to improve the durability of thecoated substrate. This tempering step is particularly advantageous forcertain ceramic substrates and specific second materials. This has beendescribed in great detail in PCT (Int'l) Application Publication No. WO2021/028035, the entire content of which is incorporated herein byreference, in particular with regard to any disclosure relating to thesepreferred material combinations and the advantages and effects of thetempering.

A substrate and a coating thereon (and optionally further additionallylayers) offer the advantage to use a rather specific and potentiallymore expensive material for the coating, which can particularly easilybe manipulated by a laser or a focused particle beam. The substrate, inthis case, merely provides a durable base material for the coating layerwhich, in fact, encodes the information. Using a substrate and a coatingalso allows for providing an optical contrast (or other contrasts whichmay be read out by a suitable reading device) between spots of coatingbeing present and spots of no coating, where the substrate material maybe read out. This effect is described in detail for ceramic substratesin the above-referenced '035 Publication, the content of which is alsoin this regard incorporated herein by reference.

If a stack of even more material layers is used and if the plurality ofrecesses have different depths the material differences betweensubsequent layers may be employed in combination with the recess depthin order to encode further information by, e.g., creating a coloreffect. This is described in great detail in the above-referenced '418Publication, the content of which is hereby incorporated by reference,in particular with regard to these facts just mentioned. Accordingly,the present invention further relates to a method for storage ofinformation comprising the steps of providing a substrate; coating thesubstrate with two or more layers of different second materials beingdifferent from the material of the substrate; and creating a pluralityof recesses in the layers of the second materials by using a laserand/or a focused particle beam in order to encode information in thelayers of the second materials; wherein the plurality of recesses havedifferent shapes and/or sizes and different depths and extend intodifferent layers of the two or more layers and wherein each shape and/orsize and each depth corresponds to a predefined value of information.Again, the coated substrate may optionally be tempered before and/orafter information encoding to improve the durability of the coatedsubstrate. As outlined above, this is particularly advantageous in casecertain ceramic substrates and certain second materials are being used.

If two or more layers are coated on a substrate, it is preferred thatthe two or more layers each have a thickness smaller than 1 μm,preferably smaller than 100 nm and more preferably smaller than 10 nm.It is also preferred that the two or more layers comprise a metal layerand a metal oxide layer, wherein the metal element of the metal layerand the metal element of the metal oxide layer are preferably identical.

For all of the three alternatives of this first aspect of the presentinvention it is preferred that the recesses of different shape have adifferent shape in a cross-section perpendicular to the depth direction.Alternatively, or additionally, the recesses of different size have adifferent size in a cross-section perpendicular to the depth direction,preferably a different cross-sectional area. Thus, the different shapesand/or sizes may be most efficiently imaged or read out along an axisperpendicular to the substrate or coating surface.

Preferably, the plurality of recesses are arranged in a regular 2Dpattern. A particularly preferred regular 2D pattern is a rectangular,preferably square, matrix or array with a single recess or no recessbeing present per rectangle or square of said matrix or array. Such anarrangement allows for an increased storage density in comparison to,e.g., optical discs which require a certain track pitch thus eliminatinga substantial portion of the overall surface for storage of data. Apreferred regular 2D pattern having even further enhanced storagedensity is a hexagonal pattern.

The plurality of recesses may have at least 2, at least 3, at least 4,at least 5, at least 6, at least 7, at least 8, at least 16 or at least32 different shapes and/or sizes and/or depths with each shape and/orsize and/or depth corresponding to a predefined value of information. Ofcourse, the number of different sizes need not correspond to the numberof different shapes and/or depths. For example, if an ellipsoidal shapeis chosen for each recess, there may be four different orientations ofsaid ellipsoid, two differently sized ellipsoids and eight differentdepths, which allows for encoding 6 different bits of information or 64different values of information per recess. Consequently, the storagedensity, in this example, is increased by a factor of six as compared toa pattern of recesses having a single shape, size and depth only.

Of course, the different shapes need not be created by differentlyoriented recesses of the same base shape such as an ellipsoid. Rather,it is also envisaged to use, e.g., a square recess, a circular recessand, e.g., a line recess with different orientations. Moreover, it isalso possible to use two or more different recesses which differ both insize and shape. For example, if the laser being used in the inventivemethod creates, in general, circular recesses, one may generate either asingle circular recess or two circular recesses next to each othergenerating a slightly elongated shape, which differs in size and shapefrom the circular recess.

Preferably, each recess is formed by one or more pulses of the laserand/or focused particle beam. The shape of each recess may be controlledby one or a combination of optical proximity control, polarizationablation, variable shaped beam technology or by two or more circularrecesses overlapping each other in certain orientations. For example,one may create a triangular shape by positioning three recesses in atriangular pattern next to each other with sufficient overlap tobasically generate a triangular recess. Similarly, four recesses may begenerated to create a rectangular or square recess. Two or more recessesmay also form a line-shaped recess which could have differentorientations.

The size of each recess may be controlled by one or a combination ofpulses, intensity levels or size of the focal spot. For example, if alaser beam having a focus with a conical shape is being used, severalpulses and/or pulses of higher intensity level will not only achieve arecess having increased depth but, due to the conical shape, at the samelead to a larger cross-section of the recess at its upper end (at thesurface). If depth and size of each recess need be controlledindependently from each other, the focal spot may be manipulated inorder to chance the size of the recess. As it will be cumbersome andtime-consuming to adapt the focal spot of the laser beam for each singlerecess, it is preferred to first create all recesses having a firstsize, to then manipulate the focal spot in order to subsequently createall recesses having a second different size.

The depth of each recess may be controlled by one or a combination ofthe following parameters: energy of the pulses, duration of the pulses,number of pulses of the laser and/or focused particle beam.

If different depths are employed, the minimum depth difference betweenthe plurality of recesses is at least one 1 nm, preferably at least 10nm, more preferably at least 30 nm, more preferably at least 50 nm, evenmore preferably at least 70 nm and most preferably at least 100 nm. Theminimum depth difference between the plurality of recesses is at most 5μm, preferably at most 1 μm, more preferably at most 500 nm, morepreferably at most 300 nm, even more preferably at most 200 nm and mostpreferably at most 100 nm.

In general, any material may be utilized for the substrate of thepresent invention, which may either be manipulated by a laser and/orparticle beam or which is sufficiently stable and durable to form a basematerial to be coated as discussed above. It is, however, particularlypreferred that the substrate is a ceramic substrate as explained in theabove-referenced '035 Publication.

The ceramic substrate preferably comprises an oxidic ceramic substrate.The ceramic substrate preferably comprises at least 90%, preferably atleast 95% by weight of one or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂,ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃ or any other oxidic ceramic material. Theceramic substrate preferably comprises a glassy transparent ceramicmaterial or a crystalline ceramic material. The ceramic substratepreferably comprises one or a combination of: sapphire (Al₂O₃), silica(SiO₂), zirconium (Zr(SiO₄)), ZrO₂. Particularly preferred substratematerials, which inter alia allow for achieving substrate thicknessessmaller than 200 μm are silicon oxide, aluminum oxide, boron oxide,sodium oxide, potassium oxide, lithium oxide, zinc oxide and magnesiumoxide. Reference is in this regard made to EP 4044182.

The ceramic substrate preferably comprises a non-oxidic ceramicsubstrate. The ceramic substrate preferably comprises at least 90%,preferably at least 95% by weight of one or a combination of a metalnitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN,HfN, BN, a metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC,B₄C, SiC, a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂,HfB₂, WB₂, WB₄; and a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi,WSi₂, PtSi, Mg₂Si or any other non-oxidic ceramic material. Preferably,the ceramic substrate comprises one or a combination of Ni, Cr, Co, Fe,W, Mo or other metals with a melting point above 1,400° C. Preferably,the ceramic material and the metal form a metal matrix composite.Preferably, the metal amounts to 5-30% by weight, more preferably 10-20%per weight of the ceramic substrate. It is particularly preferred thatthe ceramic substrate comprises WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Moand/or SiC/Co—Ni—Mo.

The second material(s) may be any material which may be suitablymanipulated by a laser and/or particle beam in order to generate therecesses of the present invention. However, certain materials areparticularly preferred for the second material(s), in particular if usedin combination with the above-mentioned ceramic materials of thesubstrate, as outlined in detail in the above-referenced '035Publication. Preferably, the second material(s) comprise(s) at least oneof a metal such as Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn,Mg, Hf, Mo, V or a ceramic material such as a metal nitride such as CrN,CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN, a metalcarbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC, an metaloxide such as Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃, ametal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆,ThB₂, HfB₂, WB₂, WB₄,or a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi,Mg₂Si, or any other ceramic material, preferably wherein the secondmaterial comprises CrN, Cr₂O₃ and/or CrAlN.

The technique of how the recesses are created is not decisive for thepresent invention as long as the size and shape as well as optionallythe depth may be well-controlled by said technique. Preferably, creatingthe recesses comprises locally heating the surface (of the substrate orthe coating) to at least a melting temperature and/or a decompositiontemperature of the second material(s), preferably to a temperature of atleast 3,000° C., more preferably at least 3,200° C., even morepreferably at least 3,500° C. and most preferably at least 4,000° C. Itis also preferred to create recesses by treating the surface of the(coated) substrate with a femtosecond-laser in order to cause so-calledCoulomb explosions leading to material ablation.

The layer of the second material and/or the two or more layers ofdifferent second materials preferably has/have a thickness no greaterthan 5 μm, preferably no greater than 2 μm, more preferably no greaterthan 1 μm, even more preferably no greater than 100 nm and mostpreferably no greater than 10 nm.

Creating the recesses may comprise locally heating, decomposing,oxidizing, ablating and/or vaporizing the surface of the substrate orcoating.

As outlined above, it may be beneficial to temper the coated substratebefore and/or after information encoding to improve the durability ofthe coated substrate as again described in detail in theabove-referenced '035 Publication. This is particularly preferred if aceramic substrate is being used. This tempering preferably generates asintered interface between the ceramic substrate and the layer of thesecond material or the two or more layers of different second materials.Preferably, said sintered interface comprises at least one element fromboth the substrate material and the second material(s). Tempering maytake place in an oxygen atmosphere which may cause oxidation of at leasta top most sub-layer of the layer of the second material(s).

The inventive method may allow for achieving an increased storagecapacity. Preferably, areas of the (coated) substrate comprise at least1 Megabyte of information per cm², more preferably at least 10 Megabytesof information per cm², even more preferably at least 100 Megabytes ofinformation per cm², even more preferably at least 1 Gigabyte ofinformation per cm² and most preferably at least 10 Gigabytes ofinformation per cm².

The invention according to this first aspect also relates to acorresponding information storage medium resulting from the methodsdiscussed above. Accordingly, the present invention relates to aninformation storage medium comprising a substrate, wherein the surfaceof the substrate comprises a plurality of recesses encoding theinformation on the information storage medium, wherein the plurality ofrecesses have different shapes and/or sizes and wherein each shapeand/or size corresponds to a predefined value of information. Asdiscussed above the plurality of recesses may also have different depthswith each depth corresponding to a predefined value of information.

The present invention further relates to an information storage mediumcomprising a substrate coated with a layer of a second material, whereinthe second material is different from the material of the substrate,wherein the layer of the second material comprises a plurality ofrecesses encoding information on the information storage medium, whereinthe plurality of recesses have different shapes and/or sizes and whereineach shape and/or size corresponds to a predefined value of information.The information storage medium may further comprise an optional sinteredinterface between the substrate and the layer of the second materialwherein said sintered interface comprises at least one element from boththe substrate material and the second material. This sintered interfaceis particularly preferred if the substrate comprises a ceramic material.As mentioned above, the plurality of recesses may also have differentdepths with each depth corresponding to a predefined value ofinformation.

The present invention further relates to an information storage mediumcomprising a substrate coated with two or more layers of differentsecond materials, wherein the second materials are different from thematerial of the substrate, wherein the information storage mediumcomprises a plurality of recesses encoding information on theinformation storage medium, wherein the plurality of recesses havedifferent shapes and/or sizes and different depths and extend intodifferent layers of the two or more layers and wherein each shape and/orsize and each depth corresponds to a predefined value of information.Again, an optional sintered interface may be present between thesubstrate and the bottommost layer of the two or more layers, whereinthe sintered interface comprises at least one element from both thesubstrate material and the material of the bottommost layer.

The optional and preferred features discussed above in the context ofthe inventive method may, of course, also employed for the inventiveinformation storage media.

If two or more layers are coated on the information storage medium, itis preferred that the two or more layers each have a thickness smallerthan 1 μm, preferably smaller than 100 nm and more preferably smallerthan 10 nm. It is also preferred that the two or more layers comprise ametal layer and a metal oxide layer, wherein the metal element of themetal layer and the metal element of the metal oxide layer arepreferably identical.

It is also preferred that the recesses of different shapes have adifferent shape in a cross-section perpendicular to the depth directionand/or wherein recesses of different size have a different size in across-section perpendicular to the depth direction, preferably adifferent cross- sectional area.

It is also preferred that the plurality of recesses have at least two,preferably at least three, more preferably at least four, even morepreferably at least five, even more preferably at least six, even morepreferably at least seven, even more preferably at least eight, evenmore preferably at least sixteen and most preferably at least thirty-twodifferent shapes and/or sizes and/or depths and wherein each shapeand/or size and/or depth corresponds to a predefined value ofinformation.

Furthermore, it is preferred that the minimum depth difference betweenthe plurality of recesses is at least 1 nm, preferably at least 10 nm,more preferably at least 30 nm, more preferably at least 50 nm, evenmore preferably at least 70 nm, and most preferably at least 100 nm.Preferably, the minimum depth difference between the plurality ofrecesses is at most 5 μm, preferably at most 1 μm, more preferably atmost 500 nm, more preferably at most 300 nm, even more preferably atmost 200 nm, and most preferably at most 100 nm.

The ceramic substrate preferably comprises an oxidic ceramic. Theceramic substrate preferably comprises at least 90%, preferably at least95%, by weight of one or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂,MgO, Cr₂O₃, Zr₂O₃, V₂O₃ or any other oxidic ceramic material. Theceramic substrate preferably comprises a glassy transparent ceramicmaterial or a crystalline ceramic material. The ceramic substratepreferably comprises one or a combination of: sapphire (Al₂O₃), silica(SiO₂), zirconium (Zr(SiO₄)), ZrO₂. Particularly preferred substratematerials, which inter alia allow for achieving substrate thicknessessmaller than 200 μm are silicon oxide, aluminum oxide, boron oxide,sodium oxide, potassium oxide, lithium oxide, zinc oxide and magnesiumoxide.

Further, the ceramic substrate preferably comprises a non-oxidicceramic. The ceramic substrate preferably comprises at least 90%,preferably at least 95%, by weight of one or a combination of a metalnitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN,HfN, BN; a metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC,B₄C, SiC; a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB, HfB₂,WB₂, WB₄ and a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂,PtSi, Mg₂Si, or any other non-oxidic ceramic material. Preferably, theceramic substrate comprises one or a combination of Ni, Cr, Co, Fe, W,Mo or other metals with a melting point above 1,400° C. Preferably, theceramic material and the metal form a metal matrix composite.Preferably, the metal amounts to 5-30% by weight, more preferably 10-20%per weight of the ceramic substrate. It is particularly preferred thatthe ceramic substrate comprises WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Moand/or SiC/Co—Ni—Mo.

Preferably, the second material(s) comprise(s) at least one of a metalsuch as Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, Vor a ceramic material such as a metal nitride such as CrN, CrAlN, TiN,TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN, a metal carbide such asTiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC, a metal oxide such asAl₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃, a metal boridesuch as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄, or a metalsilicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si, or anyother ceramic material. Particularly preferred second materials compriseCrN, Cr₂O₃ and/or CrAlN.

Preferably, the information storage medium comprises an oxide layer ontop of the second material(s), wherein the oxide layer preferablycomprises one or more oxides of the second material or of the materialof the topmost layer of the two or more layers of different secondmaterials.

Preferably, the layer(s) of the second material(s) has/have a thicknessno greater than 10 μm, preferably no greater than 3 μm, even morepreferably no greater than 1 μm, even more preferably no greater than100 nm, even more preferably no greater than 10 nm.

Areas of the information storage medium preferably comprise at least 1Megabyte of information per cm², preferably at least 10 Megabytes ofinformation per cm², more preferably at least 100 Megabytes ofinformation per cm², even more preferably at least 1 Gigabyte ofinformation per cm², even more preferably at least 10 Gigabytes ofinformation per cm².

The melting temperature of the, preferably ceramic, substrate, thesintered layer and the layer of the second material or the two or morelayers of different second materials is preferably greater than 1,000°C., preferably greater than 1,200° C., more preferably greater than1,300 ° C. The melting temperature of the, preferably ceramic, substrateis preferably equal to or greater than the melting temperature of thelayer of the second material or the two or more layers of differentsecond materials.

The inventive information storage medium may be used for long-terminformation storage, in particular if a ceramic material is employed forthe substrate. Accordingly, the present invention further relates to ause of the information storage medium discussed above for long-terminformation storage, wherein the information storage medium ispreferably stored for a period of at least 10 years, preferably at least100 years, more preferably at least 1,000 years, more preferably atleast 10,000 years and even more preferably at least 100,000 years.

The present invention further relates to a method of decodinginformation encoded on the information storage medium as discussedabove. The method of decoding comprises the steps of providing theinformation storage medium discussed above, measuring the shape and/orsize, and optionally the depth, of at least a subset of the plurality ofrecesses, and decoding the values of information corresponding to themeasured shapes and/or sizes and optionally the measured depths.

Preferably, measuring the shape and/or size and optionally the depth, isperformed using a laser beam and/or a focused article beam such as anelectron beam.

Preferably, measuring the shape and/or size and optionally the depth isbased on one or a combination of: interference, reflection, absorption,ellipsometry, frequency comb technique, fluorescence microscopy such asSTED or STORM, structured illumination, super-resolution microscopy,optical coherence tomography, ptychography, scanning electronmicroscopy, digital (immersion) microscopy (using reflected ortransmitted light). The high resolution achieve by the optical techniquemay be further enhanced by pattern recognition utilizing well-known AItechniques.

According to the first aspect of the present invention discussed above,a third dimension (in addition to the x and y coordinates of therecesses) is being used in order to increase storage capacity, whereinsaid “third dimension” may be either the depth of the recess or the type(shape and/or size) of the recess. By combining depth and type, evenmore information may be stored within the same surface area.

According to another, second aspect of the present invention discussedfurther below, storage capacity is increased by changing the pattern ofthe recesses (within the x-y-surface) in order to allow for morepermutations of recess patterns.

The invention according to this second aspect (an exemplary embodimentof which is shown schematically in FIG. 5 a and FIG. 5 b ) inter aliarefers to a method for storage of information comprising the steps ofproviding a substrate; and creating a plurality of recesses in a surfaceof the substrate by using a laser and/or a focused particle beam inorder to encode information on the substrate. The plurality of recessesare located at a subset of first predetermined positions and/or at asubset of second predetermined positions, wherein the firstpredetermined positions define a regular pattern with a center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses, wherein the second predetermined positions define a regularpattern with a center-to-center distance between directly adjacentpositions corresponding to at least 75% and to at most 150% of themaximum cross-sectional dimension of the recesses, wherein thecenter-to-center distance between any of the first predeterminedpositions and the directly adjacent second predetermined position issmaller than 75% of the maximum cross-sectional dimension of therecesses, and wherein for each pair of directly adjacent first andsecond predetermined positions only one of these directly adjacent firstand second predetermined positions is occupied by a recess.

In essence, this aspect of the present invention is based on the idea toutilize inter-matrix positions in order to achieve additional potentialpositions for recesses to encode additional bits of data. For example,if one starts out with a square matrix as the subset of firstpredetermined positions, one would typically allow one circular recessto be present for each square unit cell of the square matrix in such amanner that adjacent circular recesses just or barely touch each other.This allows for a certain number of permutations for one up to N²recesses being present if the side length of the square matrix allowsfor N recesses to be placed adjacent to each other. Yet, the presentinvention, in addition, utilizes a subset of second predeterminedpositions also defining a regular pattern with a center-to-centerdistance between directly adjacent position corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses. But the center-to-center distance between any of the firstpredetermined positions and a directly adjacent second predeterminedposition is smaller than 75% of the maximum cross-sectional dimension ofthe recesses. In other words, the subset of second predeterminedpositions is shifted with respect to the subset of first predeterminedpositions. For example, said shift may correspond to one half of thecenter-to-center distance between directly adjacent positions of thefirst predetermined positions. Accordingly, an additional position for apotential recess is offered exactly between two positions for recessespreviously present. Since it may not be possible to distinguish, duringdecoding, between a situation with two adjacent first predeterminedpositions being occupied and the situation with those two positions anda third position in between being occupied, the present inventionsuggests to utilize only either of these two scenarios. Accordingly, foreach pair of directly adjacent first and second predetermined positionsonly one of these directly adjacent first and second predeterminedpositions is occupied by a recess. In terms of decoding, this merelyrequires that one can distinguish between a circular recess beingpresent at a certain first position and said same recess being presentat position shifted by, e.g., one radius of said circular recess.Utilizing this technique, the storage capacity per surface area may besubstantially increased.

Since there may be a certain variation in the center-to-center distancesand/or the maximum cross-sectional dimension of the recesses, theabove-mentioned criterium may apply only on average, i.e. the firstpredetermined positions may define a regular pattern with an averagecenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the average maximumcross-sectional dimension of the recesses, and the second predeterminedpositions may define a regular pattern with an average center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the average maximum cross-sectional dimensionof the recesses, wherein the center-to-center distance between any ofthe first predetermined positions and the directly adjacent secondpredetermined position is smaller than 75% of the average maximumcross-sectional dimension of the recesses.

In order to be on the safe side one may also define a strictercriterium. Accordingly, the first predetermined positions may define aregular pattern with a largest center-to-center distance betweendirectly adjacent positions corresponding to at least 75% and to at most150% of the smallest maximum cross-sectional dimension of the recesses,and the second predetermined positions may define a regular pattern witha largest center-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the smallestmaximum cross-sectional dimension of the recesses, wherein thecenter-to-center distance between any of the first predeterminedpositions and the directly adjacent second predetermined position issmaller than 75% of the smallest maximum cross-sectional dimension ofthe recesses.

In order to allow for a reliable distinction between first and secondpositions, it is preferred that the center-to-center distance betweenany of the first predetermined positions and the directly adjacentsecond predetermined position is smaller than the 70%, preferablysmaller than 65%, more preferably smaller than 60% and even morepreferably smaller than 55% of the (smallest or average) maximumcross-sectional dimension of the recesses.

In order to ensure that adjacent recesses are properly separated fromone another, it is further preferred that the first predeterminedpositions define a regular pattern with a (average) center-to-centerdistance between directly adjacent positions corresponding to at least105%, preferably at least 110%, more preferably at least 115% of the(average or largest) maximum cross-sectional dimension of the recesses,wherein the second predetermined positions define a regular pattern witha center-to-center distance between directly adjacent positionscorresponding to at least 105%, preferably at least 110%, morepreferably at least 115% of the (average or largest) maximumcross-sectional dimension of the recesses.

Similar to the first aspect of the present invention discussed above,this second aspect of the present invention may also be utilized forrecesses in a substrate or recesses in a coating.

Accordingly, the present invention further relates to a method forstorage of information (an exemplary embodiment of which is shownschematically in FIG. 6 ) comprising the steps of providing a substrateand coating the substrate with a layer of a second material differentfrom the material of the substrate as well as creating a plurality ofrecesses in a surface of the layer of the second material by using alaser and/or a focused particle beam in order to encode information inthe layer of the second material. The plurality of recesses are locatedat a subset of first predetermined positions and/or at a subset ofsecond predetermined positions, wherein the first predeterminedpositions define a regular pattern with a center-to-center distancebetween directly adjacent positions corresponding to at least 75% and toat most 150% of the maximum cross-sectional dimension of the recesses,wherein the second predetermined positions define a regular pattern witha center-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the maximumcross-sectional dimension of the recesses, wherein the center-to-centerdistance between any of the first predetermined positions and a directlyadjacent second predetermined position is smaller than 75% of themaximum cross-sectional dimension of the recesses, and wherein for eachpair of directly adjacent first and second predetermined positions onlyone of these directly adjacent first and second predetermined positionsis occupied by a recess.

Of course, the above-discussed features as to limits and averages applyas well to this method.

If the above discussed scenario with two directly adjacent recesses onthe one hand and a third recess in between on the other hand isoptically (or by means of any other technique) distinguishable duringdecoding, even further positions may be utilized for additionalrecesses. Accordingly, the present invention further relates to a methodfor storage of information comprising the steps of providing a substrateand creating a plurality of recesses in a surface of the substrate byusing a laser and/or a focused particle beam in order to encodeinformation on the substrate. The plurality of recesses are located at asubset of first predetermined positions and/or at a subset of secondpredetermined positions, wherein the first predetermined positionsdefine a regular pattern with a center-to-center distance betweendirectly adjacent positions corresponding to at least 75% and to at most150% of the maximum cross-sectional dimension of the recesses, whereinthe second predetermined positions define a regular pattern with acenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the maximumcross-sectional dimension of the recesses, wherein the center-to-centerdistance between any of the first predetermined positions and a directlyadjacent second predetermined position is smaller than 75% of themaximum cross-sectional dimension of the recesses.

Again, this concept may be used for recesses in a substrate or recessesin a coating. Thus, the present invention further relates to a methodfor storage of information comprising the steps of providing a substrateand coating the substrate with a layer of a second material differentfrom the material of the substrate and creating a plurality of recessesin a surface of the layer of the second material by using a laser and/ora focused particle beam in order to encode information in the layer ofthe second material. The plurality of recesses are located at a subsetof first predetermined positions and/or at a subset of secondpredetermined positions, wherein the first predetermined positionsdefine a regular pattern with a center-to-center distance betweendirectly adjacent positions corresponding to at least 75% and to at most150% of the maximum cross-sectional dimension of the recesses, whereinthe second predetermined positions define a regular pattern with acenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the maximumcross-sectional dimension of the recesses, wherein the center-to-centerdistance between any of the first predetermined positions and a directlyadjacent second predetermined position is smaller than 75% of themaximum cross-sectional dimension of the recesses.

Again, the preferred ranges for the distances discussed above apply tothis alternative as well.

In the latter two cases, it is preferred that the regular pattern of thefirst predetermined positions defines a pattern of voids if all of thefirst predetermined positions are occupied, wherein each of the secondpredetermined positions completely covers a void if occupied. It isfurther preferred that each center of each of the second predeterminedpositions corresponds to a center of one of the voids.

As mentioned above, the regular pattern of the first predeterminedpositions may be, e.g., a square pattern. However, other regularpatterns such as, e.g., a hexagonal pattern may also be employed.

Preferably, the shape of the recesses is cylindrical or cone-like.Cylindrical or substantially cylindrical recesses are particularlypreferred because they make decoding less prone to errors, in particularin case of a rather dense pattern. It is thus preferred that thecross-sectional area of the recess at the bottom of the recesscorresponds to at least 50%, more preferably at least 70%, morepreferably at least 80%, even more preferably at least 90% of thecross-sectional area of the recess at the top or edge of the recess.Preferably, the recesses are created using a laser beam with a Besselbeam shape as this allows for creating cylindrical or substantiallycylindrical recesses.

The preferred and optional features discussed above with respect to thefirst aspect of the present invention may be analogously employed forthis second aspect of the present invention. In particular, the conceptof inter-matrix positions for recesses may also be combined withrecesses having different shape and/or size and/or depth.

The ceramic substrate preferably comprises an oxidic ceramic. Theceramic substrate preferably comprises at least 90%, preferably at least95%, by weight of one or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂,MgO, Cr₂O₃, ZrO₃, V₂O₃ or any other oxidic ceramic material. The ceramicsubstrate preferably comprises a glassy transparent ceramic material ora crystalline ceramic material. The ceramic substrate preferablycomprises one or a combination of: sapphire (Al₂O₃), silica (SiO₂),zirconium (Zr(SiO₄)), ZrO₂. Particularly preferred substrate materials,which inter alia allow for achieving substrate thicknesses smaller than200 μm are silicon oxide, aluminum oxide, boron oxide, sodium oxide,potassium oxide, lithium oxide, zinc oxide and magnesium oxide.

Further, the ceramic substrate preferably comprises a non-oxidicceramic. The ceramic substrate preferably comprises at least 90%,preferably at least 95%, by weight of one or a combination of a metalnitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN,HfN, BN; a metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC,B₄C, SiC; a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB, HfB₂,WB₂, WB₄ and a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂,PtSi, Mg₂Si, or any other non-oxidic ceramic material. Preferably, theceramic substrate comprises one or a combination of Ni, Cr, Co, Fe, W,Mo or other metals with a melting point above 1,400° C. Preferably, theceramic material and the metal form a metal matrix composite.Preferably, the metal amounts to 5-30% by weight, more preferably 10-20%per weight of the ceramic substrate. It is particularly preferred thatthe ceramic substrate comprises WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Moand/or SiC/Co—Ni—Mo.

The second material(s) preferably comprise(s) at least one of a metalsuch as Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, Vor a ceramic material such as a metal nitride such as CrN, CrAlN, TiN,TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN, a metal carbide such asTiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC, an metal oxide such asAl₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃, a metal boridesuch as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄ or a metalsilicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si or anyother ceramic material. Preferably, the second material(s) comprise(s)CrN, Cr₂O₃ and/or CrAlN.

The layer of the second material and/or the two or more layers ofdifferent second materials preferably has/have a thickness no greaterthan 5 μm, preferably no greater than 2 μm, more preferably no greaterthan 1 μm, even more preferably no greater than 100 nm and mostpreferably no greater than 10 nm.

The plurality of recesses preferably have different depths, wherein eachdepth corresponds to a predefined value of information.

The minimum depth difference between the plurality of recesses ispreferably at least 1 nm, preferably at least 10 nm, more preferably atleast 30 nm, more preferably at least 50 nm, even more preferably atleast 70 nm, and most preferably at least 100 nm. The minimum depthdifference between the plurality of recesses is preferably at most 5 μm,preferably at most 1 μm, more preferably at most 500 nm, more preferablyat most 300 nm, even more preferably at most 200 nm, and most preferablyat most 100 nm.

The present invention according to this second aspect further relates toan information storage medium. The information storage mediumcompromises a substrate, wherein the surface of the substrate comprisesa plurality of recesses encoding information on the information storagemedium, wherein the plurality of recesses are located at a subset offirst predetermined positions and at a subset of second predeterminedpositions, wherein the first predetermined positions define a regularpattern with a center-to-center distance between directly adjacentpositions corresponding to at least 75% and to at most 150% of themaximum cross-sectional dimension of the recesses, wherein the secondpredetermined positions define a regular pattern with a center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses, wherein the center-to-center distance between any of the firstpredetermined positions and a directly adjacent second predeterminedposition is smaller than 75% of the maximum cross-sectional dimension ofthe recesses, and wherein for each pair of directly adjacent first andsecond predetermined positions only one of these directly adjacent firstand second predetermined positions is occupied by a recess.

Again, this concept can be extended to a substrate with a coating and toan inter-matrix code with overlap as discussed above with regard to themethod.

Accordingly, the present invention according to this second aspectfurther relates to an information storage medium comprising a substratecoated with a layer of a second material and an optional sinteredinterface between the substrate and the layer of the second material,wherein the second material is different from the material of thesubstrate, wherein the optional sintered interface comprises at leastone element from both the substrate material and the second material andwherein the layer of the second material comprises a plurality ofrecesses encoding information on the information storage medium. Theplurality of recesses are located at a subset of first predeterminedpositions and at a subset of second predetermined positions, wherein thefirst predetermined positions define a regular pattern with acenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of the maximumcross-sectional dimension of the recesses, wherein the secondpredetermined positions define a regular pattern with a center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses. The center-to-center distance between any of the firstpredetermined positions and a directly adjacent second predeterminedposition is smaller than 75% of the maximum cross-sectional dimension ofthe recesses, and wherein for each pair of directly adjacent first andsecond predetermined positions only one of these directly adjacent firstand second predetermined positions is occupied by a recess.

Of course, the preferred and optional features discussed above withrespect to the methods according to the second aspect of the presentinvention as well as with respect to the methods and media according tothe first aspect of the present invention, may also be employed for theinformation media according to this second aspect of the presentinvention.

It is preferred that the regular pattern of the first predeterminedpositions is a square pattern. However, other patterns such as, e.g., ahexagonal pattern may also be employed.

The shape of the recesses is preferably cylindrical or substantiallycylindrical or cone-like. As mentioned above, it is particularlypreferred that the cross-sectional area at the bottom of a recesscorresponds to at least 50%, preferably at least 60%, more preferably atleast 70%, even more preferably at least 80% and particularly preferablyat least 90% of the cross-sectional area at the top or edge of therecess.

The ceramic substrate preferably comprises an oxidic ceramic. Theceramic substrate preferably comprises at least 90%, preferably at least95%, by weight of one or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂,MgO, Cr₂O₃, Zr₂O₃, V₂O₃ or any other oxidic ceramic material. Theceramic substrate preferably comprises a glassy transparent ceramicmaterial or a crystalline ceramic material. The ceramic substratepreferably comprises one or a combination of: sapphire (Al₂O₃), silica(SiO₂), zirconium (Zr(SiO₄)), ZrO₂. Particularly preferred substratematerials, which inter alia allow for achieving substrate thicknessessmaller than 200 μm are silicon oxide, aluminum oxide, boron oxide,sodium oxide, potassium oxide, lithium oxide, zinc oxide and magnesiumoxide.

Further, the ceramic substrate preferably comprises a non-oxidicceramic. The ceramic substrate preferably comprises at least 90%,preferably at least 95%, by weight of one or a combination of a metalnitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN,HfN, BN; a metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC,B₄C, SiC; a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB, HfB₂,WB₂, WB₄ and a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂,PtSi, Mg₂Si, or any other non-oxidic ceramic material. Preferably, theceramic substrate comprises one or a combination of Ni, Cr, Co, Fe, W,Mo or other metals with a melting point above 1,400° C. Preferably, theceramic material and the metal form a metal matrix composite.Preferably, the metal amounts to 5-30% by weight, more preferably 10-20%per weight of the ceramic substrate. It is particularly preferred thatthe ceramic substrate comprises WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Moand/or SiC/Co—Ni—Mo.

Preferably, the second material(s) comprise(s) at least one of a metalsuch as Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, Vor a ceramic material such as a metal nitride such as CrN, CrAlN, TiN,TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN, a metal carbide such asTiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC, an metal oxide such asAl₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃, a metal boridesuch as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄ or a metalsilicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si or anyother ceramic material. Preferably, the second material(s) comprise(s)CrN, Cr₂O₃ and/or CrAlN.

The layer of the second material and/or the two or more layers ofdifferent second materials preferably has/have a thickness no greaterthan 5 μm, preferably no greater than 2 μm, more preferably no greaterthan 1 μm, even more preferably no greater than 100 nm and mostpreferably no greater than 10 nm.

The plurality of recesses preferably have different depths, wherein eachdepth corresponds to a predefined value of information.

The minimum depth difference between the plurality of recesses ispreferably at least 1 nm, preferably at least 10 nm, more preferably atleast 30 nm, more preferably at least 50 nm, even more preferably atleast 70 nm, and most preferably at least 100 nm and/or wherein theminimum depth difference between the plurality of recesses is at most 5μm, preferably at most 1 μm, more preferably at most 500 nm, morepreferably at most 300 nm, even more preferably at most 200 nm, and mostpreferably at most 100 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail inthe following text with reference to preferred exemplary embodimentswhich are illustrated in the attached drawings, in which:

FIG. 1 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention;

FIG. 2 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention;

FIG. 3 a depicts the 16 code elements of a regular 2×2 square -matrixcode both schematically (top) and as a transmission micrograph at amagnification of 100×taken from a sample (bottom);

FIG. 3 b depicts the 16 code elements of FIG. 3 a and the additional 19code elements possible with an inter-matrix code without overlapaccording to the present invention, both schematically (top) and as atransmission micrograph at a magnification of 100×taken from a sample(bottom);

FIG. 4 shows the principle of an inter-matrix code with overlap in caseof a 4×4 square matrix code according to a preferred embodiment of thepresent invention, both schematically (top) and as a transmissionmicrograph at a magnification of 150×taken from a sample (bottom);

FIG. 5 a shows techniques to define a reference depth for a matrix codewithout overlap, both schematically (top) and as a transmissionmicrograph at a magnification of 150×taken from a sample (bottom);

FIG. 5 b shows techniques to define a reference depth for a matrix codewith overlap, both schematically (top) and as a transmission micrographat a magnification of 150×taken from a sample (bottom);

FIG. 6 depicts an example of various shapes and sizes of recessesaccording to a preferred embodiment of the present invention, bothschematically (top) and as a transmission micrograph at a magnificationof 50×taken from a sample (bottom);

FIG. 7 shows how various shapes of recesses may be achieved usingmultiple circular recesses according to a preferred embodiment of thepresent invention, both schematically (top) and as a transmissionmicrograph at a magnification of 100×taken from a sample (bottom);

FIG. 8 schematically shows alphanumeric and Chinese character sets basedon square segments in comparison with a dot-matrix and inter-matrix coderelated to information capacity in bits;

FIG. 9 illustrates an estimation of the potential maximum storagecapacity considering spatial frequency, phase shift and amplitude ofrecesses.

FIG. 10 a depicts 80 of the code elements possible with an inter-matrixcode with overlap according to the present invention, both schematically(top) and as a transmission micrograph at a magnification of 100×takenfrom a sample (bottom); and

FIG. 10 b depicts all of the code elements possible with an inter-matrixcode with overlap according to the present invention as a transmissionmicrograph at a magnification of 100×taken from a sample.

In principle, identical parts are provided with the same reference signsin the figures.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention. The information storage medium comprises a, preferablyceramic, substrate 150 coated with a layer of a second material 170, thesecond material 170 being different from the material of the substrate150. As mentioned above, a sintered interface (not shown) may be presentbetween the substrate 150 and the layer of the second material 170 dueto the optional tempering process, in particular in case the substrate150 comprises a ceramic material. The layer of the second material 170comprises a plurality of recesses 10 (four of which are shown exemplary)having different depths, wherein each depth corresponds to a predefinedvalue of information. In the embodiment shown in FIG. 1 , four bits ofinformation can be encoded. For example, the smallest depth of a recess10 (or, alternatively, a surface without any recess at all) maycorrespond to the value of information of code “0000”. The largest depthof a recess 10 extending, for example, all the way through the secondlayer 170 to the substrate 150 may correspond to the value ofinformation or code “1111”. Analogously, each of the intermediate depthscorresponds to a specific predefined value of information or code aswell. While the depth difference between subsequent codes is shown inFIG. 1 to be constant, this does not necessarily to be the case.

Of course, the 4-bit code shown in FIG. 1 is only one specific example.Depending on the thickness of the second layer 170 and the depthdifferences of the various recesses 10 which can be both reliablymanufactured for encoding and reliably measured for decoding more orless bits may be encoded.

Techniques for manufacturing such a substrate with a coating and forcreating recesses of various depths within such a coating (or thesubstrate only) are described in great detail in the above-referenced'035 and '418 Publications, the entire contents of which areincorporated herein by reference, in particular with respect to saidtechniques.

The recesses 10 are merely shown schematically in FIG. 1 . However, aselucidated in detail above, the plurality of recesses of the presentapplication have different shapes and/or sizes in order to encodeinformation, wherein each shape and/or size corresponds to a predefinedvalue of information. FIG. 6 depicts an example of various shapes andsizes of recesses according to a preferred embodiment of the presentinvention, both schematically (top) and as generated in a sample(bottom). In particular, the first four sketches of the top row of FIG.6 show three differently sized circular recesses (as well as no recesson the very left), which are clearly distinguishable from each other. Inthis example, three information values may be encoded by means of thesize of the recess. Similarly, two bits (corresponding to four values ofinformation) may be encoded by using different shapes of recesses asshown, e.g., in the last four sketches of the top row of FIG. 6 or thefirst four sketches in the bottom row of FIG. 6 . These exemplarydifferent shapes may also be realized by a single base shape (such as anellipsoid or a triangle in the example of FIG. 6 ) being differentlyoriented. Of course, the various shapes shown in FIG. 6 may also becombined. For example, one may encode ten different values ofinformation with one of the circular recesses and of the square recessesas shown in the top and bottom rows of FIG. 6 , the four differentlyoriented triangles and the four differently oriented ellipses. Inaddition, or alternatively, differently sized recesses may be used toencode information.

As should be evident from the above summary of the first aspect of thepresent invention, the various recesses need not to have differentdepths as shown in FIG. 1 , but may also only differ in shape and/orsize as, for example, shown in FIG. 6 . Moreover, the recesses may notbe present in a layer of a second material 170 as shown in FIG. 1 , butmay also be present in a substrate material (without any coating).

If, however, recesses of different depths are employed, it isadvantageous to provide a reference depth which allows, during decoding,to measure, e.g., the difference in height between the substrate orcoating surface and, e.g., the bottom of each recess. For this purpose,the various recesses arranged, for example, in a square pattern may beprovided within a reference frame 1 surrounding the entire pattern asschematically shown in FIG. 5 a , where 16 circular recesses areschematically arranged in a square pattern. Alternatively, the referenceframe may be present at only one, two or three sides of the pattern.Thus, an optical decoding system may measure, for example, the distancebetween a reference point of the optical decoding system and thereference frame on the one hand and the distance between said referencepoint and the bottom of each recess on the other hand. Thus, thedistance between the bottom of each recess and the reference frame maybe evaluated.

Alternatively, or in addition, other areas of the pattern where norecess is present, may be utilized in order to provide a reference. Forexample, the “void” 2 shown in FIG. 5 a may be used to define areference height.

FIG. 2 schematically depicts a cross section through an informationstorage medium according to a further preferred embodiment of thepresent invention. The information storage medium comprises a,preferably ceramic, substrate 150 coated with four layers 171 to 174 ofdifferent second materials being different from the material of thesubstrate 150. Again, a sintered interface (not shown) may be present atleast between the substrate 150 and the bottommost layer 171 of the fourlayers. The sintered interface may comprise at least one element fromboth the substrate material and the material of the bottommost layer171. Similar to the embodiment shown in FIG. 1 , the information storagemedium of the embodiment shown in FIG. 2 comprises a plurality ofrecesses 10 encoding information on the information storage medium,wherein the plurality of recesses 10 have different depths and whereineach depth corresponds to a predefined value of information. Again, 16different depths are shown in FIG. 2 corresponding to a 4-bit code.

However, different from the embodiment shown in FIG. 1 , in case of theembodiment shown in FIG. 2 four different bits are encoded (by means ofdifferent depths) in each of the four layers 171 to 174. If the fourlayers 171 to 174 are made from different materials, the opticalresponse of each layer may be different. This allows for achieving highaccuracy during decoding because the depth information achieved may becorrelated with, for example, the optical response.

Of course, more or less than four layers of different second materialsmay be present depending on the number of bits to be encoded.

One particularly preferred example for the multi-layer coating shown inFIG. 2 is a two-layer coating with a metal layer 171 being coated on thesubstrate 150 and a metal oxide layer (of the same metal) 172 beingcoated on the metal layer 171. If such a two-layer coating isilluminated with incident white light, a part of the incident light isreflected at the oxide layer, whereas another part of the incident lightis refracted into the oxide layer and reflected at the oxide/metalinterface, as explained in the above-referenced '418 Publication. Thelight beam having been reflected at the oxide layer and the light beamhaving being reflected at the metal layer can be in phase, which leadsto a visible colour, or out of phase, which does not yield said colourto be visible. Accordingly, a certain colour (which depends on theindices of refraction of both the oxide layer and the metal layer andthe thickness of the oxide layer) is visible wherever the oxide layer ispresent, yet is invisible if the depth of a certain recess leads todestructive interference at this particular spot, as explained in theabove-referenced '418 Publication.

As elucidated in detail in the above description of the first aspect ofthe present invention, the concept of these various recesses extendinginto different layers of the two or more layers can be advantageouslycombined with the idea of the present invention to provide a pluralityof recesses having different shapes and/or sizes as exemplary shown inFIG. 6 .

FIG. 3 schematically shows the concept of the inter-matrix positions ofrecesses according to the second aspect of the present invention. FIG. 3a schematically depicts the 16 code elements of a regular 2×2 squarematrix code with the circles representing positions of recesses on saidsquare matrix. These 16 permutations allow for encoding four bits ofinformation. FIG. 3 b schematically depicts the 16 code elements of FIG.3 a (upper portion) and the additional 19 code elements (lower portion)possible with an inter-matrix code without overlap according to thepresent invention.

The bottom portion of FIG. 3 b shows the same 2×2 square matrix as FIG.3 a . However, now inter-matrix positions are occupied by recesses,which inter-matrix positions correspond to positions being arrangedexactly and symmetrically in between two positions of the regular squarematrix according to FIG. 3 a . This allows for encoding 19 further codeelements leading to a total of 35 code elements, which corresponds to5.13 bits. In other words, utilizing the inter-matrix positionssubstantially enhances the storage capacity of said 2×2 square matrix.

The scheme shown in FIG. 3 is based on the requirement that no adjacentrecesses overlap each other in order to allow for an accurate readoutwithout fault. However, if such overlap is accepted (e.g. due to animproved resolution during decoding), the storage capacity may befurther increased. This is illustrated in FIG. 4 , which, on the farleft, schematically depicts the 16 bits of a regular 4×4 square matrixcode with the 16 circles representing the 16 possible positions ofrecesses on said square matrix without overlap. Now, if one accepts twoadjacent circular recesses to overlap by one radius of a recess, one mayshift each position of each recess by one radius to the right. Thisyields 12 additional positions for recesses (within the original square)as indicated in the second sketch of FIG. 4 . These 12 additionalpositions correspond to 12 further bits of information. Similarly, onemay shift each position of each recess by one radius to the bottom asindicated in the third sketch of FIG. 4 or to the right and the bottomas indicated on the far right in FIG. 4 . Thus, an additional 12 bitsand 9 bits may be gained. In total, the 4×4 inter-matrix code withoverlap allows for encoding a total of 49 bits as compared to the 16bits of the regular 4×4 square matrix code.

As indicated in FIG. 4 , this scheme may be generalized to anyrectangular M×N matrix. Due to the three possibilities to shift saidpattern discussed above, such a matrix allows for encoding M×Nbits+N×(M−1) bits+M×(N−1) bits+(N−1)×(M−1) bits=[4MN−2(N+M)+1] bits ascompared to the MN bits of the regular M×N rectangular matrix code.

As an intermediate alternative between the two above-discusses extremesone may also choose to only one particular form of overlap. For example,one may combine the positions shown in the far left of FIG. 4 with thoseshown in the far right of FIG. 4 only. In terms of decoding this onlyrequires that the pattern shown in the far left of FIG. 4 may bereproducibly generated in such a manner as to ensure that each void inbetween each square pattern of four recesses, where no material has beenremoved (i.e., a “void” corresponds to a protrusion of material on thesurface of the information storage medium), is maintained. If one canprecisely control the size and position of each recess such that it isguaranteed that a void is always present in between a square arrangementof four recesses, a further code element can be created by placing afurther recess on said very void. These are the positions shown in thefar right of FIG. 4 . If such overlap is allowed, an M×N matrix providesan additional (M−1)×(N−1) positions for further recesses, leading to atotal of 2MN−M−N+1 bits (or N²+(N−1)² bits in case of the N×N squarematrix).

An example of such an inter-matrix code with overlap is shownschematically in FIG. 5 b for a 4×4 square matrix of recesses with 7 ofthe 9 voids being occupied by additional recesses. As discussed abovewith respect to FIG. 5 a , the reference frame 1 or one of the voids 2,which is not occupied, may be used as a reference height for measuringthe depth of each recess.

As discussed above with respect to FIG. 6 , the plurality of recesses ofthe present application may have different shapes in order to encodeinformation, wherein each shape corresponds to a predefined value ofinformation. The bottom part of FIG. 6 shows an experimental realizationof the various shapes depicted schematically in the top part of FIG. 6 .For the example, a ceramic substrate having a size of 10×10 mm andconsisting of 100 μm thick sapphire substrate (Al₂O₃) was coated with acoating of CrN having a thickness of 100 nm by means of physical vapordeposition (PVD). Circular recesses having a diameter of about 1 μm(i.e. dimensions much smaller than the shapes to be achieved) wereablated from the coating using a 200 femtosecond laser at a wavelengthof 515 nm in order to generate certain geometric shapes without anycoating.

The resulting information storage medium was imaged with an OlympusBX-51 at a magnification of 50×. As can be seen by comparing the top andbottom parts of FIG. 6 the various shapes can be reproducibly generatedwith great accuracy and the difference in shape is easily discernably bynaked eye. Rather than piecing the shown shapes together by multiplecircular recesses one may also achieve such shapes by using laser beamshaping, e.g., with the help of a spatial light modulator (SLM).

FIG. 7 more clearly shows the technique discussed in the context of FIG.6 for achieving various shapes of recesses using multiple circularrecesses according to a preferred embodiment of the present invention,both schematically (top) and as generated in a sample (bottom). Forexample, the arrangements of two overlapping circular recesses in fourdifferent patterns shown in the first four sketches of the top row ofFIG. 7 may yield the four ellipsoids being differently oriented as shownin the bottom row of FIG. 6 . Similarly, three overlapping circularrecesses in the far right of the bottom row of FIG. 7 may yield thetriangular shape shown in the far right of the top row of FIG. 6 . Asindicated in the remaining arrangements in the bottom row of FIG. 7 ,said triangular shape may also be oriented differently. The bottom partof FIG. 7 shows an experimental realization of the various shapesdepicted schematically in the top part of FIG. 7 . For the example, aceramic substrate having a size of 10×10 mm and consisting of 100 μmthick sapphire substrate (Al₂O₃) was coated with a coating of CrN havinga thickness of 100 nm by means of PVD. Circular recesses having adiameter of about 1 μm were ablated from the coating using a 200femtosecond laser at a wavelength of 515 nm.

The resulting information storage medium was imaged with an OlympusBX-51 at a magnification of 100×. Each individual dot is 1 μm indiameter. As can be seen by comparing the top and bottom parts of FIG. 7the various shapes can be reproducibly generated with great accuracy andthe difference in shape is easily discernably by naked eye. While onemay, in case of this particular example, even distinguish the variouscircular recesses forming each shape, this is apparently not necessaryas long as the resulting shapes of the interconnecting recesses arestill distinguishable from each other by a suitable imaging and/ordetection technique.

In FIG. 8 , alphanumeric and Chinese character sets based on squaresegments are compared with a dot-matrix and inter-matrix code related toinformation capacity in bits.

With a 5×7 square matrix an alphanumeric character set (UTF-8) with 256different characters (8 bits) can be displayed and deciphered by thehuman eye. With an 8×8 square matrix approximately 27,550 Chinesecharacters (14.75 bits) can be displayed and deciphered by experiencedChinese readers. By contrast, a 5×7 or 8×8 square matrix with circularrecesses can achieve about 4.4 or 4.3 times more combinations that couldbe displayed and recognized by a digital reading system. Even furtherincrease of storage capacity can be achieved by using inter-matrixpositions in accordance with the present invention, which enable up toM×N+M×(N−1)+N×(M−1)+(M−1)×(N−1) different patterns as exemplified inFIG. 4 . According to this formula the information capacity of a 5×7matrix can be increased from 35 bits to 117 bits or by a further factor3.3. This ratio increases to a factor 3.5 for the larger 8×8 matrixenabling 225 bits for inter- matrix coding. For larger matrices, thisratio approximates 4 as can be observed by the example of a 32×32matrix, which enables 1,024 bits as a regular dot matrix and 3,969 bitsin the inter-matrix regime. While it might be difficult to employ eachof these patterns for reliable discrimination since the overlappingmatrixes require a very high special resolution of the optical systemand the detector, even a more practical approach of using, e.g., onlyabout 3,000 bits would provide a tremendous advantage over prior arttechniques.

A more general way of estimating the potential maximum storage capacityper mm² can be derived from FIG. 9 , assuming a 2-dimentional surfacedetermined by x- and y-axes in which the special frequency (lines permm), the number of phase-shift positions along these axes and one halfof the number of depth-levels of the recesses determine the capacity permm². The factor ½ is required to convert the number of depth levels intobits, since one bit equals actually two different depth levels.

Data Storage Density≤(F _(x) ·|P _(x)|)·(F _(y) ·|P_(y)|)·(1/2|A|)·Bits/mm²

FIG. 10 a depicts 80 of the code elements of a regular 2×2 square matrixpossible with an inter-matrix code with overlap according to the presentinvention, both schematically (top) and as generated in a sample(bottom). As is evident from said figure, the use of overlapdramatically increases the number of potential code elements as comparedto the 35 possible code elements without overlap (see FIG. 3 b ). Ofcourse, this tremendous number of code elements may only be utilized ifthese code elements can on the one hand be precisely and reproduciblygenerated and on the other hand correctly read out by a correspondingreading system with a sufficiently small error rate.

In order to prove that this is, in fact, possible, an experiment hasbeen performed generating each and every possible code element for aregular 2×2 square matrix possible with an inter-matrix code withoverlap. For the example, a ceramic substrate having a size of 10×10 mmand consisting of 100 μm thick sapphire substrate (Al₂O₃) was coatedwith a coating of CrN having a thickness of 100 nm by means of PVD.Circular recesses having a diameter of about 1 μm were ablated from thecoating using a 200 femtosecond laser at a wavelength of 515 nm.

The resulting information storage medium was imaged with an OlympusBX-51 at a magnification of 100×. Each individual dot is 1 μm indiameter. The result of said experiment is shown in FIG. 10 b . As canbe seen (also by comparing the top and bottom parts of FIG. 10 a ) thevarious code elements can be reproducibly generated with great accuracyand the difference in shape is easily discernably even by naked eye.

1-83. (canceled)
 84. A method for storage of information comprising thesteps of: providing a substrate of a first material; coating thesubstrate with a layer of a second material different from the firstmaterial, wherein the coated substrate comprises a plurality of firstpredetermined positions and a plurality of second predeterminedpositions; and creating a plurality of recesses in the layer of thesecond material by using a laser and/or a focused particle beam in orderto encode information in the layer of the second material; wherein theplurality of recesses occupy a subset of the first predeterminedpositions and/or a subset of the second predetermined positions, whereinthe first predetermined positions define a regular pattern with acenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of a maximumcross-sectional dimension of the recesses, wherein the secondpredetermined positions define a regular pattern with a center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses, and wherein a center-to-center distance between any pair ofone of the first predetermined positions and a directly adjacent secondpredetermined position is smaller than 75% of the maximumcross-sectional dimension of the recesses.
 85. The method of claim 84,wherein for each pair of directly adjacent first and secondpredetermined positions only one of the first and second predeterminedpositions is occupied by a recess.
 86. The method of claim 84, whereinthe regular pattern of the first predetermined positions defines apattern of voids if each of the first predetermined positions isoccupied by one of the recesses, and wherein each of the secondpredetermined positions, if occupied by a recess, completely covers avoid.
 87. The method of claim 86, wherein each center of each of thesecond predetermined positions corresponds to a center of one of thevoids.
 88. The method of claim 84, wherein the center-to-centerdistances between the directly adjacent positions in the regular patternof the first and second predetermined positions corresponds to at least95% of the maximum cross-sectional dimension of the recesses.
 89. Themethod of claim 84, wherein the center-to-center distances between thedirectly adjacent positions in the regular pattern of the first andsecond predetermined positions corresponds to at most 130% of themaximum cross-sectional dimension of the recesses.
 90. The method ofclaim 84, wherein the center-to-center distance between any pair of oneof the first predetermined positions and the directly adjacent secondpredetermined position is smaller than 60% of the maximumcross-sectional dimension of the recesses.
 91. The method of claim 84,wherein the first material is a ceramic.
 92. The method of claim 84,wherein the second material comprises one or a combination of Cr, Co,Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, or V.
 93. Themethod of claim 84, wherein the second material comprises one or acombination of a metal nitride; a metal carbide; a metal oxide; a metalboride; or a metal silicide.
 94. An information storage medium,comprising: a substrate of a first material coated with a layer of asecond material different from the first material, wherein the coatedsubstrate comprises a plurality of first predetermined positions and aplurality of second predetermined positions; and a sintered interfacebetween the substrate and the layer of the second material; wherein thesintered interface comprises at least one element from both the firstmaterial and the second material, wherein the layer of the secondmaterial comprises a plurality of recesses encoding information, whereinthe plurality of recesses occupy a subset of the first predeterminedpositions and/or a subset of the second predetermined positions, whereinthe first predetermined positions define a regular pattern with acenter-to-center distance between directly adjacent positionscorresponding to at least 75% and to at most 150% of a maximumcross-sectional dimension of the recesses, wherein the secondpredetermined positions define a regular pattern with a center-to-centerdistance between directly adjacent positions corresponding to at least75% and to at most 150% of the maximum cross-sectional dimension of therecesses, and wherein a center-to-center distance between any pair ofone of the first predetermined positions and a directly adjacent secondpredetermined position is smaller than 75% of the maximumcross-sectional dimension of the recesses.
 95. The information storagemedium of claim 94, wherein for each pair of directly adjacent first andsecond predetermined positions only one of the first or secondpredetermined positions is occupied by a recess.
 96. The informationstorage medium of claim 94, wherein the center-to-center distancesbetween the directly adjacent positions in the regular pattern of thefirst and second predetermined positions corresponds to at least 85% ofthe maximum cross-sectional dimension of the recesses.
 97. Theinformation storage medium of claim 94, wherein the center-to-centerdistances between the directly adjacent positions in the regular patternof the first and second predetermined positions corresponds to at most140% of the maximum cross-sectional dimension of the recesses.
 98. Theinformation storage medium of claim 94, wherein the center-to-centerdistance between any pair of one of the first predetermined positionsand the directly adjacent second predetermined position is smaller than70% of the maximum cross-sectional dimension of the recesses.
 99. Theinformation storage medium of claim 94, wherein the first material is aceramic.
 100. The information storage medium of claim 94, wherein thesecond material comprises one or a combination of Cr, Co, Ni, Fe, Al,Ti, Si, W, Zr, Ta,Th, Nb, Mn, Mg, Hf, Mo, or V.
 101. The informationstorage medium of claim 94, wherein the second material comprises one ora combination of a metal nitride; a metal carbide; a metal oxide; ametal boride; or a metal silicide.
 102. The information storage mediumof claim 94, wherein the layer of the second material has a thickness nogreater than 1 μm.
 103. The information storage medium of claim 94,wherein the coated substrate comprises the plurality of the firstpredetermined positions and the plurality of the second predeterminedpositions, and wherein the plurality of recesses occupy a subset of thefirst predetermined positions and a subset of the second predeterminedpositions.